This invention relates to an exhaust system for a motor vehicle, and particularly to a system for measuring the amount of oxygen in exhaust gas emitted by an internal-combustion engine.
In order to comply with exhaust gas regulations, it may be necessary to determine tne residual oxygen content in the exhaust gas of internal-combustion engines and, by using this value, regulate the fuel/air ratio on the intake side. Probes are generally used to determine the oxygen content in the exhaust gas that work according to the principle of an oxygen concentration chain. They normally consist of a pipe consisting of a solid electrolyte, sucn as ZrO.sub.2 which is acted upon by the exhaust gas on one side and by a gas with a known oxygen content on the other side.
Heretofore, since tne solid electrolyte requires a working temperature of about 400.degree. to 800.degree. C., the probe had been screwed into the exhaust gas pipe through a receiving opening at a point where tne exnaust gases have the corresponding temperature in order to heat tne probe to its working temperature. However, it was found that the exhaust gas temperature also fluctuates considerably because of the considerably changing loads affecting the engine so that the danger exists that tne probe may either be overheated at full load or cooled during part load or coasting operations, and it may thus lose its effectiveness. It has therefore been tried to heat probes in order to make their working temperature more uniform. Although a better result is obtained by means of these heated probes, the danger still exists that the probe will be overheated during full-load operation or overcooled during coasting operation of the engine due to the design of the heating means unless the temperature of the probe is adapted to different exhaust gas temperatures by means of a very fast reacting and therefore extremely costly temperature control system.
One object of the present invention is therefore to find an exhaust gas pipe for internal-combustion engines having a receiving opening for a heated probe for determining the oxygen content of tne exhaust gases where an overheating or overcooling of the probe is reliably avoided without need for a costly temperature control system.
According to the present invention, a system is provided for measuring the amount of oxygen in exhaust gas emitted by an internal-combustion engine. The system includes an exhaust pipe for conducting engine exhaust gas and probe means for determining tne oxygen content of engine exhaust gases. The exhaust pipe has a side wall formed to include an aperture for continuously conducting a portion of the engine exhaust gas conducted through the pipe. The system further includes support means for positioning the probe means at a location outside of the exhaust pipe in proximity to the side wall aperture so that the probe means is exposed to the engine exhaust gas portion discharged through the aperture and for collecting exhaust gas in a space surrounding the probe means. The aperture is sized to permit exhaust gas collected in the support means to be reintroduced into the exhaust pipe so that collected exhaust gas will be exchanged continuously with exhaust gas flowing through tne exhaust pipe.
Preferred embodiments of the present invention provide an exhaust gas pipe with a recess or a blind hole, the side walls of which are located outside the exhaust gas flow. The overheated probe is mounted in this recess. The opening of the recess in the direction of the exhaust gas flow is smaller than the cross-section of the recess, cross-sections of from 1 to 200 mm.sup.2 having been especially suitable. When tne cross-section of the opening becomes smaller than 1 mm.sup.2, there will no longer be a sufficient gas exchange with the exhaust gas flowing in the exhaust gas pipe, and the determination of the composition of the exhaust gas will take place too slowly. When the opening of the recess or the blind hole in the direction of the exhaust gas flow is larger than 200 mm.sup.2, the gas exchange with the exhaust gas flowing in the exhaust gas pipe is so brisk that overheating as well as overcooling of the probe may occur. Cross-sections of from 3 to 50 mm.sup.2 for the opening of the recess are preferred. In the case of these small cross-sections, a sufficient exchange with the gas filling located in the recess will still take place because of the pulsations of the exhaust gas in the exhaust gas pipe.
In addition, the depth of the recess must be dimensioned in such a way that the point of the probe is set back with respect to the opening to the exhaust gas flow in order to protect it from the attack by particles located in the exhaust gas and which enter the recess through the recess opening. A setback between the point of the probe and the recess opening also contributes to making the probe temperature more uniform. Good results are obtained when the depth of tne recess is dimensioned in such a way that the point of the probe is set back with respect to the opening to the exhaust gas flow by approximately 5 to 90 mm. A useful rule is that the depth of the recess should be the larger, or the point of the probe should be the further set back with respect to the opening to the exhaust gas flow, the larger the opening is in the direction of the exhaust gas.
It was also found to be advantageous for the clearance of the recess to be dimensioned in such a way that the distance of the wall of the recess to the wall of the probe is about 1 to 10 mm. In the case of such a design of the recess, the gas volume located in the recess is relatively small so that the probe works without much delay caused by a gas volume to be changed that is too large. The wall of the probe in this case is the actual wall of the probe, i.e., the solid-electrolyte pipe, in which case protective devices that may be arranged around the solid-electrolyte pipe, such as a sheet-metal covering or a wire basket, for preventing unintended contact with the sensitive ceramics of the solid electrolyte, are not taken into account with respect to the dimensioning.
The manufacturing of the exhaust gas pipe may, for example, take place by providing the exhaust gas pipe with a bore which represents the opening of the recess to the exhaust gas flow and by welding or soldering a hat-shaped recess or blind hole provided with a thread for receiving the probe at this point onto the exhaust gas pipe. In addition, the recess may be provided witn a thermic insulation.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with he accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.